Sizergh Castle is a stately home with garden and estate at Helsington in Cumbria, England, about 4 miles (6 km) south of Kendal. Located in historic Westmorland, the castle is a Grade I listed building. While remaining the home of the Hornyold-Strickland family, the castle with its garden and estate is in the care of the National Trust.

In 2016 the Sizergh estate was included in the newly extended Lake District National Park.

The earliest part of the building is a tower of fourteenth or fifteenth century date.

Some of the early furnishings date from the time of Walter Strickland (1516–1569) who married Alice Tempest in 1560. She made inventories of the house after her husband's death. These mention three oak armchairs and three chests still in the house.

There are oak-panelled interiors, including the Inlaid Chamber, where the panelling is inlaid with floral and geometric patterns in pale poplar and dark bog-oak. The contents of the Inlaid Chamber were sold to the Victoria and Albert Museum in the 1890s and it was displayed as a reconstructed period room. The return of the panelling to its original location at Sizergh was advocated by among others Mark Girouard, an authority on England's country houses. The panelling returned in 1999 under a long-term loan. In 2017 it was reported that transfer of ownership to the National Trust had been made formal.

The bargeboards on the gables probably date from the seventeenth century.

The Castle contains a variety of paintings, including the following:

The Deincourt family owned this land from the 1170s. On the marriage of Elizabeth Deincourt to Sir William de Stirkeland in 1239, the estate passed into the hands of what became the Strickland family, who owned it until it was gifted to the National Trust in 1950 by Lieutenant Commander Thomas Hornyold-Strickland, 7th Count della Catena, R.N., a grandson of the 1st Baron Strickland.